1. Field of the Invention
The present invention generally relates to an echo canceler and, more particularly, to an acoustic echo canceler for use in a telephone set (hereunder referred to simply as a “handsfree telephone set”) having a handsfree conversation function.
2. Description of the Related Art
Handsfree telephone sets are preferably used in various places and situations and have a high frequency of use because of no necessity for picking up a handset at each telephone call. However, generally, there has been a problem of echo generated during a handsfree conversation mode by feeding back and inputting voices, which are outputted by a speaker, to a microphone. It is, thus, necessary for favorably talking over a telephone to cancel such echo. Hitherto, many pieces of research on an echo canceler for canceling echo have been conducted. Particularly, an echo canceler for canceling acoustic echo generated in a system employing a speaker and a microphone is sometimes referred to as an “acoustic echo canceler”.
FIG. 1 is a block diagram illustrating the configuration of a conventional acoustic echo canceler employed in a handsfree telephone set.
In FIG. 1, reference numeral 1, 2, 3, 4, 5, 6, and 7 designate a speaker amplifier, a speaker, a microphone, a microphone amplifier, an adaptive filter, an adder, and an acoustic echo path, respectively. Incidentally, a main body (not shown) of a telephone set is placed on the right side in FIG. 1.
A voice signal received by the main body of the telephone set of a called party from a calling party is amplified by the speaker amplifier 1. Then, the amplified voice signal is outputted from the subsequent speaker 2. A part of an output of the speaker 2 is inputted to the microphone 3 through the acoustic echo path 7. Then, a signal obtained by the microphone 3 therefrom is amplified by the microphone amplifier 4. Subsequently, this amplified signal (S-IN) is inputted to one of input terminals of the adder 6. On the other hand, an output signal of the adaptive filter 5 for generating a pseudo echo signal in response to the input of the received voice signal as an input signal (R-IN) is inputted to the other input terminal of the adder 6. Then, an output signal (S-OUT) of the adder 6 is used as another input signal (namely, an error signal) to the adaptive filter 5, and also transmitted from the main body of the telephone set to the calling party as a transmitted voice signal.
The adaptive filter 5 estimates the frequency characteristic (or response) and the phase characteristic of the echo path, that is, the frequency characteristic and the phase characteristic, which are synthesized from those of the speaker amplifier 1, the speaker 2, the acoustic echo path 7, the microphone 3, and the microphone amplifier 4. Moreover, the adaptive filter 5 sequentially updates filter coefficients so that the filter coefficients of the adaptive filter 5 have the same characteristics as the estimated characteristics. Consequently, the received voice signal having passed through the adaptive signal becomes a signal (that is, a pseudo echo signal), which is close to an echo signal inputted to the microphone 3. Thus, an echo signal inputted to a positive input terminal (+) of the adder 6 is canceled by inputting the pseudo echo signal to a negative input terminal (−) thereof.
FIG. 2 illustrates an ordinary constitution of the adaptive filter 5.
In FIG. 2, reference character x(k) designates an input signal to the adaptive filter 5. Signals outputted from delay elements D, through which signals x(k),  pass, are designated by x(k−1), x(k−2), , x(k−L+1), respectively. A total sum of results of multiplying each of the signals x(k−1), x(k−2), , x(k−L+1) by a corresponding one of filter coefficients w1(k), w2(k), , wL(k) is represented by an output signal y(k) of the adaptive filter 5. Incidentally, “L” denotes a total number of filter coefficients.
A filter of such a constitution is known as an FIR (Finite Impulse Response) filter. The FIR filter is enabled by setting the filter coefficients thereof to have an arbitrary frequency characteristic and a given phase characteristic. The filter coefficients w1(k), w2(k), , wL(k) are updated according to an adaptation algorithm in such a manner as to minimize the magnitude of an error signal e(k). Generally, the filter coefficients in an initial state are initialized by being set at zero. A learning identification method is known as an example of the adaptation algorithm.
An adaptation operation of the adaptive filter 5 is performed according to the signal R-IN and the error signal. The adaptive filter 5 updates the filter coefficients in such way as to minimize the magnitude of the error signal. However, the error signal includes background noise to be inputted to the microphone 3 and also includes thermal noise of the microphone amplifier 4. The presence of these noise components hinders the adaptive filter 5 from performing the adaptation operation so as to make the frequency characteristic and the phase characteristic approximate to those of the echo path. Thus, the difference between the pseudo echo, which is obtained by letting the received voice signal through the adaptive filter 5, and the signal S-IN increases. Consequently, the presence of the noise components becomes a cause of the drawback that an echo cancellation amount, namely, an amount of echo cancellation is insufficient.
Furthermore, the frequency characteristics and the phase characteristics estimated by the adaptive filter 5 are those of the speaker amplifier 1, the speaker 2, the acoustic echo path 7, the microphone 3, and the microphone amplifier 4. Among these, the frequency characteristic and the phase characteristic of the acoustic echo path 7 vary with a place where the handsfree telephone set is set up. The frequency characteristic and the phase characteristic of each of the remaining elements, that is, the speaker amplifier 1, the speaker 2, the microphone 3, and the microphone amplifier 4 do not change.
FIG. 3 shows an example of the characteristic of the speaker. Further, FIG. 4 shows an example of the characteristic of the microphone.
Each of these graphs shows the characteristic obtained by letting the voice signal through a corresponding amplifier (not shown) and a PCM CODEC (not shown), which complies ITU-T G. 712, in need thereof for measurement in addition to a corresponding one of the single speaker unit and the single microphone unit. As is seen from FIG. 4, the characteristic of the microphone is represented by a relatively flat and simple curve. In contrast, the characteristic of the speaker shown in FIG. 3 is represented by a complex curve. Incidentally, the frequency characteristics of this amplifier and the PCM CODEC are nearly flat in a voice band. Consequently, most of operations of estimating the frequency characteristic and the phase characteristic of the echo path by the adaptive filter 5 are carried out for estimating the frequency characteristics and the phase characteristics of the acoustic echo path 7 and the speaker 2.
As described above, the frequency characteristics and the phase characteristics to be estimated by the adaptive filter 5 are substantially those of the acoustic echo path 7 and the speaker 2. Among those, the frequency characteristic and the phase characteristic of the speaker 2 do not change at each handsfree conversation. However, in the conventional acoustic echo canceler, the operation of estimating the frequency characteristic and the phase characteristic of the speaker 2 is performed together with the operation of estimating the frequency characteristic and the phase characteristic of the acoustic echo path 7. Thus, when a noise is superposed on a voice signal obtained according to the complex characteristics of the speaker, the frequency characteristics and the phase characteristics of the echo path are not completely estimated. Especially, the frequency characteristic and the phase characteristic of the speaker 7 are insufficiently estimated. Consequently, the conventional acoustic echo canceler has drawbacks in that the operation of canceling echo is unstable, and that the echo cancellation amount is insufficient.